1. Field of the Invention
This invention relates to an illuminating optical device for uniformly illuminating an object, and in particular to an illuminating optical device suitable for an exposure apparatus for manufacturing semiconductive chips such as IC, LSI and VLSI.
2. Related Background Art
As an illuminating optical device used in an exposure apparatus of this type, a construction using an optical integrator is well known. In such a construction, however, the degree of achievement of the uniformity of illumination distribution is not always satisfactory and further, the uniformity of the illumination distribution outside a surface to be irradiated is remarkably inferior. Therefore, where such an illuminating optical device is used as the illuminating system of a projection type exposure apparatus (a stepper), there has been the difficulty that the resolving power and the depth of focus of the projecting lens are deteriorated. In order to eliminate such a difficulty, an illuminating optical device of a construction in which fly-eye type integrators are used in two stages is patented as U.S. Pat. No. 4,497,015 issued to the applicant.
The construction disclosed in this U.S. Pat. No. 4,497,015 is shown in FIG. 14 of the accompanying drawings. In FIG. 14, a light source 1 is disposed so that the light emitting point thereof coincides with the vicinity of a first focus of an elliptical mirror 2, and a condensing point (the light source image) is formed in the vicinity of a second focus (a surface A1). A light flux passed through the condensing point is substantially collimated by a positive lens 3 and enters a first fly-eye type integrator 4. Two positive lenses 5 and 6 are disposed between the first fly-eye type integrator 4 and a second fly-eye type integrator 7, and the first and second fly-eye type integrators 4 and 7 are constructed so that their entrance surfaces (surfaces B1 and B2) are conjugate with each other and their exit surfaces (surfaces A2 and A3) are conjugate with each other. The light flux which has left the second fly-eye type integrator 7 uniformly illuminates a surface 0 to be irradiated through a field lens 8 and a condenser lens 9.
What has been described above is the basic construction disclosed in the aforementioned U.S. Patent, and in FIG. 14, surfaces which are in a conjugate relation with the light source 1 are designated by reference characters A1, A2 and A3, and surfaces which are in a conjugate relation with the surface 0 to be irradiated are denoted by reference characters B1 and B2.
In the prior-art device of the construction as shown in FIG. 14, fly-eye type integrators are used in two stages and therefore, an increase in the number of fly-eye lens elements constituting the fly-eye type integrators is unavoidable, and the increase in the number of parts has led to the problem that the steps of manufacture and the cost of manufacture are increased.
Also, where use is made of a light source of very high output such as an excimer laser, there has been the fear that optical elements are destroyed by the heat generation at the condensing point.